1. Field of the Invention
The present invention relates to high performance multi-pair data cables, and more particularly, to multi-pair cables using different twist lay lengths and pair proximity control to meet category six performance specifications.
2. Discussion of Related Art
As is known in the art, cables formed from twisted pairs of insulated conductors are used to transfer communication signals between, for example, components of a local area network (LAN) such as computers, telephones, and other devices. The TIA/EIA-568A specification sets out transmission requirements, such as, for example, maximum acceptable crosstalk, skew and impedance mismatch values between twisted pairs, for cables that are classified as Category 5 (Cat. 5) and category 6 (Cat. 6) cables. In order to meet these requirements various techniques are employed to control crosstalk between twisted pairs and skew.
Referring to FIG. 1a, there is illustrated a related art cable comprising four twisted pairs of insulated conductors 20, 22, 24, 26. Each twisted pair 20, 22, 24, 26 comprises two metallic conductors 28 each surrounded by a layer of insulation 30 and twisted together. It can be seen that due to the arrangement of the four twisted pairs 20, 22, 24, 26 there exists a central void 32 within the cable, separating non-adjacent pairs 20–26 and 22–24. According to U.S. Pat. No. 4,873,393 to Friesen et al, the twist lay length for each twisted insulated conductor pair should not exceed about forty times the outer diameter of the insulation 30 of one of the conductors 28 of the twisted pair. In addition, in order to reduce interpair crosstalk, twisted pairs with similar twist lay lengths should be located opposite one another (e.g., twisted pairs 20, 26) rather than adjacent one another (e.g., twisted pairs 20, 22). For example, the twisted pairs of the cable of FIG. 1a may have twist lay lengths such as shown below in Table 1.
TABLE 1Twist Lay LengthPair Number(inches)200.350220.680240.770260.380
As can be seen with reference to FIG. 1a and Table 1, the difference between the twist lays of twisted pairs 20 and 24, located adjacent one another, is 0.420 which is larger than the difference 0.090 between the twist lays of twisted pairs 22, 24, located opposite one another. In conventional cables such as the one illustrated in FIG. 1a, the central void 32 is relied upon to provide distance between oppositely-located twisted pairs, thereby reducing crosstalk and enabling a smaller twist delta between those pairs.
In reality, the pair arrangement in a conventional four pair cable, after assembly, is more likely to resemble the configuration shown in FIG. 1b. Rotational effects cause nesting of the twisted pairs, such that the central void 32a is substantially reduced. For this and other reasons, conventional cables such as those illustrated in FIGS. 1a and 1b may meet the requirements for Cat. 5 cables, but may not reliably meet the Cat. 6 performance requirements. In order to achieve reliable Cat. 6 cables, prior art cables generally include a central filler or cross-web (not illustrated) located in the central void 32 to further separate the twisted pairs. Alternatively, each of the twisted pairs may include an individual metallic shield disposed about the insulation layer 30.